1. Field of the Invention
This invention relates generally to the subject matter of the inorganic and biological calcium carbonate formation. More particularly, it relates to the inhibition of calcium carbonate deposition by a polysaccharide fraction obtained from calcium carbonate-forming organisms. This polysaccharide fraction has been found effective for the inhibition of inorganic or biological CaCO.sub.3 -deposition on a surface with which it is contacted.
2. Description of the Prior Art
Control of CaCO.sub.3 -encrustation and growth of calcifying organisms on surfaces in marine environments has long been recognized as a potentially solvable problem. By preventing or slowing the occurrence of these "fouling" substances in organisms, the useful lifetime of surfaces such as hulls of ships and pilings of docks can be increased. In the case of hulls of ships, prevention of fouling also has the effect of allowing the ship to move more efficiently through the water.
Historically, the problem has been approached by impregnating or coating surfaces with compounds that interfere with the metabolism of fouling organisms. For example, the use of inhibitors of carbonic anhydrase, an exzyme often involved in calcification, has been suggested for such use (Costlow, J. D., Physiological Zoology, 32:177 (1959)). More recently, inhibitors of the enzyme polyphenol oxidase, also involved in the calcification process have been shown effective as anti-fouling compounds (Turner, R. D., Symposium on Marine Biodeterioration, Naval Institute Press, Washington, D.C.). Less specific metabolic inhibitors, such as organotin compounds, are also being applied (Good, M. L., Symposium on Marine Biodeterioration, supra).
In addition, CaCO.sub.3 crystal growth occurs abiotically in most natural solutions leading to unwanted calcified deposits. For example, scale builds up anywhere in the sea where nucleation occurs, because sea water is supersaturated with respect to CaCO.sub.3 by a factor of 5 to 10-fold, allowing crystal growth to proceed spontaneously (Stumm, W. and Morgan, J. J., Aquatic Chemistry, John Wiley and Sons, Somerset, N.J. (1981)). Inorganic scales are also often encountered as unwanted deposits in pipes and boilers where supersaturation becomes a problem due to evaporative concentrations of ions. Carboxylates, such as NTA, ethylene diamine tetraacetate (EDTA) and gluconates have been used to retard or inhibit the precipitation of supersaturated solutions of calcium carbonate, although somewhat high concentrations are needed for these compounds to act as effective inhibitors. Hexametaphosphate, at 1-10 ppm concentration was found to retard scaling leading to the widespread use of polyphosphates as scale inhibitors in municipal and industrial water systems. (Monsanto's Technical Bulletin No. IC/SCS-323, Dequest 2010 Phosphonate).
In recirculating cooling water systems, calcium carbonate is generally the predominant scalant. Since cooling towers are efficient air scrubbers, this circulating water is saturated with CO.sub.2, establishing an equilibrium between bicarbonate and carbonate in solution. As the pH of the water rises, this equilibrium shifts towards carbonate. Heating also produces a shift in the dissolved inorganic carbon equilibrium to the right, producing calcium carbonate: ##STR1## Finally, calcium carbonate shows an inverse solubility trend, being less soluble at higher temperatures. All of these factors tend to produce scaling on critical heat-transfer surfaces which reduces the heat transfer efficiency, increases frequency of required cleaning and decreases the life of the system. Several of the inhibitors of the precipitation of calcium carbonate show the phonomenon of a threshold effect, e.g., the prevention of precipitation from supersaturated solutions of scalants by substoichiometric levels of inhibitors. Present mechanistic theories postulate that the threshold agent is absorbed on the growth sites of the scalant crystallite during the process of crystallization and alters the growth pattern so that the resultant scalant crystals are formed more slowly and are highly distorted. (Reddy M. M. and Nancollas, G. H., Desalination 12:61 (1973)).
A speculative model of organic matrix structure and function, based primarily on aspects of mollusk shell proteinaceous matrix biochemistry, as well as a brief review of the proteinaceous organic matrices from various other phyla was presented by Weiner, S., Traub, W. and Lowenstam, H. A., "Organic Matrix in Calcified Exoskeltons", in Biomineralization and Biological Metal Accum., pp. 205-224 (1983), Westbroek and De Jong, Eds., Reidel Publishing Co. Further characterization of the various matrical components, such as the soluble matrical fraction containing glycoprotein components can be found in Krampitz, G., Drolshagen, H., Hausle, J., and Hof-Irmscher, K, "Organic Matrices of Mollusk Shell", in Biomineral, and Biol. Metal Accum., supra, pp. 231-247 (1983), incorporated herein by reference. Calcium-binding, sulfated, high molecular weight glycoproteins have been identified in the soluble matrix of several species. In addition, this soluble fraction may also contain a number of smaller molecular weight glycoprotein components (Weiner, S., Lowenstam, H. A. and Hood, L. J., J. Exp. Mar. Biol. Ecol., 30:45-51 (1977), incorporated herein by reference). A further characterization of the amino acid sequence of soluble mollusk shell protein by peptide analysis after cleavage of the proteins on both sides of the Asp residues, showed a pattern of a repeating sequence of aspartic acids separated by either glycine or serine in an alternative manner with Asp. The repeating sequence observed is of the form (Asp-Y).sub.n -type, where Y is a single amino acid. The natural organic matrix of almost all mineralized tissues studied to date (both vertebrates and invertebrates) contain proteins which are enriched in aspartic acid (Asp) and/or glutamic acid (Glu) (Veis, A., and Perry A., Biochemistry 6:2049 (1967)); Shuttleworth, A., and Veis, A., Biochem. Biophys, Acta, 257:414 (1972)).
The (Asp-Y).sub.n -type sequence was hypothesized to be present in the organic matrices from a variety of molluscan species, such as Crassostrea virginicia, Mercenaria mercenaria, Crassostrea irredescens and Nautilus pompiliums, and suggested that these sequences played a function as a template for mineralization, although X-ray diffraction studies showed that there was a poor match between the Ca-Ca distances in the crystal lattice and the potential calcium-binding sites along the polypetide chain for this sequence (Weiner S., and Hood L., Science 19: 987 (1975); Weiner S., in The Chem. and Biol. of Mineral. Connective Tissues, Veis A., ed., pp., 517-521, Elsevier North Holland, Inc. (1981); and Weiner S. and Traub W., in Struct. Asp. of Recog. and Assembly in Biol. Macromolec. Balaban, N., Sussman, J. L., Traub, W. and Yonath, A., Eds., pp. 467-482 (1981), incorporated herein by reference).
Acknowledging that the process of CaCO.sub.3 nucleation and crystal growth itself is central to the process of encrustation by all calcifying organisms, such as barnacles, oysters, ship worms, algae and the like, Wheeler, A. P., George, J. W. and Evans, C. A., Science 212: 1397 (1981), incorporated herein by reference, made the discovery that a 170,000 dalton MW glycoprotein obtained from the proteinaceous matrix that permeates the CaCO.sub.3 of oyster shell is a very potent inhibitor, rather than an initiator of CaCO.sub.3 nucleation and crystal growth as previously throught. The 170,000 glycoprotein was identified by staining for carbohydrates and it was shown to contain 10.2% carbohydrate by weight. The molecular weight and carbohydrate content reported for the glycoprotein from oyster shell are comparable to those observed for the protein obtained from clams by Crenshaw, M. A., Biomineralization 6: 6 (1972), incorporated herein by reference.
Wheeler, A. P., and Sikes, C. S., in concurrently filed and copending application Ser. No. 563,280 entitled "Inhibition of the Formation of Inorganic or Biological CaCO.sub.3 -Containing Deposits by a Proteinaceous Fraction Obtained from CaCO.sub.3 -Forming Organisms", incorporated herein by reference, disclose a method of inhibiting the formation of CaCO.sub.3 -containing deposits with a glycoprotein-containing fraction isolated from CaCO.sub.3 -containing tissues obtained from CaCO.sub.3 -forming plants or animals. As such, the glycopeptide-like materials have been shown to have a broad range of MW ranging from 400 to 10.sup.8, and higher.
Sikes, C. S. and Wheeler, A. P., in concurrently filed and copending application Ser. No. 563,144, now U.S. Pat. No. 4,534,881, entitled "Inhibition of Inorganic or Biological CaCO.sub.3 Deposition by Poly Amino Acid Derivatives", incorporated herein by reference, further disclose a method of inhibiting the formation of inorganic or biological CaCO.sub.3 deposition by applying a synthetic amino acid polymer having a proteinaceous matrix-like structure.
Sikes, C. S. and Wheeler, A. P., in concurrently filed and copending application Ser. No. 563,145 entitled "inhibition of Inorganic or Biological CaCO.sub.3 Deposition by Synthetic Polysaccharide Derivatives", incorporated herein by reference, further disclose a method of inhibiting the formation of inorganic or biological depositions of CaCO.sub.3 by applying to a surface in contact with CaCO.sub.3 a synthetic saccharide polymer having a polysaccharide matrix-like structure.
None of the cofiled, copendingg applications by the present inventors are considered prior art to the present invention.
Coccolithophoridae are calcareous algae characterized by their ability to form a calcified cell cover consisting of calcite plates called coccoliths. The coccoliths from the species Emiliania huxleyi were sown to contain a water-soluble acid polysaccharide possessing Ca.sup.+2 -binding capacity (de Jong, E. W., Bosch, L. and Westbroek, P., Eur. J. Biochem., 70:611-621 (1976)). The polysaccharides were characterized to have a heterogeneous matrix containing uronic acid and having high affinity and low affinity sites for binding of Ca.sup.+2. (de Jong, E. W., Dam, W., Westbroek, P., and Crenshaw, M. A., in The Mech. of Mineral. in the Invertebrates and Plants, Watabe, N., and Wilbur, K. M., eds., University of S. Carolina Press, Columbia, pp. 135-153 (1976), incorporated herein by reference). The polysaccharide matrix was also shown to contain ester sulphates by incorporation of radioactive sulphated groups (de Jong, E., Van Rens, L., Westbroek, P. and Bosch, L., Eur. J. Biochem. 99:559-567 (1979), incorporated herein by reference).
An interest in further elucidating the role played by the structural parts of the polysaccharide matrix from CaCO.sub.3 -forming animals in the inhibition of CaCO.sub.3 incrustation and growth of calcifying organisms, prompted the present inventors to search for other potent and commercially useful inhibitors of said processes. This successful innovation and perfection, for the first time, of the process for the purification of new polysaccharide fractions substantially devoid of proteinaceous components, from this soluble matrix of CaCO.sub.3 -forming animal and plants, resulting in a significantly potent calcium carbonate-deposition inhibitor, now opens the possibility of using the animal- or plant-derived polysaccharide fractions for the inhibition of calcium carbonate deposition in pipes, boilers and the like, of widespread use in industrial environments, as well as for the prevention of fouling of surfaces in marine environments. The use of these highly potent polysaccharide inhibitors for the inhibition of CaCO.sub.3 deposition has heretofore been unknown in the art.